This Phase II SBIR project is to refine optimal ultrasound enhancement of the thrombolytic activity of t-PA in an acute stroke model, to the end of lowering the drug dose and thereby minimizing the risk of hemorrhage. Realizing this objective will open the market for thrombolysis in acute stroke, a market which presently remains impeded because of hemorrhage risk. The proposed work will explore 2MHz, 1MHz and 0.2MHz therapeutic ultrasound using a proprietary method to concurrently deliver diagnostic and therapeutic ultrasound through the same probe footprint. The diagnostic ultrasound will aid in aiming the therapeutic beam and then assess blood flow (i.e., recanalization) along the beam axis. The therapeutic ultrasound will enhance thrombolytic activity along the same beam axis. Instrumentation to perform blood flow/recanalization monitoring simultaneously with thrombolytic enhancement will be developed and optimized. Safety tests will be performed to determine which ultrasound protocols are free from adverse bioeffects in an appropriate animal model. This Phase II project is the extension of successfully completed Phase I aims. Phase I was comprised of three principle components: (1) the development of a new animal model of thromboembolic occlusion that greatly facilitates the study of promising therapeutic protocols, (2) the prototyping of an ultrasound device and probe for concurrent diagnostic and therapeutic insonation of vasculature containg thrombus, and (3) the testing of therapeutic efficacy in a series of protocols evaluating 2MHz and 0.2MHz ultrasound. Testing results of the last component demonstrated dramatically the efficacy of 0.2MHz ultrasound in enhancement of thrombolysis, while results for 2MHz had insufficient statistical power to make conclusions. The Phase I results confirm ultrasound enhanced thrombolysis as a major clinical advance towards the treatment of stroke. Ultrasound promises to shorten the recanalization time for stroke victims and to reduce the risk of intracerebral hemorrhage by enabling a lower dose of thrombolytie drug. Phase II will make use of the animal model of Phase I, and targets revisions to the therapeutic device of Phase I, a thorough evaluation of instrument efficacy, instrument safety, and ability to reduce thrombolytic drug dose.